Quick question - how do the lounge staff know that a person is under 18? When booking, anyone over 12 is considered an "adult", so I wouldn't have thought there would be anything obvious printed on the BP of a teenager. In fact, when you book a domestic flight, they don't even ask for your DOB, so I don't think the person's age would even be recorded in the ticketing info.
I know DOB is recorded in everyone's QFF profile, but depending on how the ticket was booked, it might not even be necessary to add the QFF number prior to travelling. For example, if Dad booked the flight with his points, there is no advantage in adding daughter's QFF number to the booking.
I realise they might also take the same approach that bar staff etc take, i.e. if someone looks like they might be under age, ask for ID. However, if that is what the system relies on, then I think it would be worth giving it a go, as she may or may not get asked.